Light emitting device and method of forming the same

ABSTRACT

A light-emitting device includes a transparent substrate, a transparent adhesive layer on the transparent substrate, a first transparent conductive layer on the transparent adhesive layer, a multi-layer epitaxial structure and a first electrode on the transparent conductive layer, and a second electrode on the multi-layer epitaxial structure. The multi-layer epitaxial structure includes a light-emitting layer. The transparent substrate has a first surface facing the transparent adhesive layer and a second surface opposite to the first surface, wherein the area of the second surface is larger than that of the light-emitting layer, and the area ratio thereof is not less than 1.6.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/626,742, filed on Jan. 24, 2007, entitled “LIGHT EMITTING DEVICE ANDMETHOD OF FORMING THE SAME,” which claims priority to Taiwan PatentApplication No. 095103659, entitled “LIGHT EMITTING DEVICE AND METHOD OFFORMING THE SAME”, filed on Jan. 27, 2006, the contents of which arehereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a light-emitting device, and moreparticularly to a light-emitting device having a multi-layer epitaxialstructure and a method of forming the same.

BACKGROUND

Light-emitting diodes have different light-emitting principles andstructures from conventional light sources and advantage of small volumeand high reliability, hence they can have versatile applications. Forexample, light-emitting diodes may form a variety of large-scalecomponents on demand to apply to indoor or outdoor displays. Therefore,the brightness enhancement is always an important issue in themanufacture of light-emitting diodes.

FIG. 1A is a schematic diagram of a conventional light-emitting diode.As shown in FIG. 1A, the light-emitting diode includes a substrate 110,a multi-layer epitaxial structure 130 having a light-emitting layer 131on the substrate 110, and a reflective layer 150 between the multi-layerepitaxial structure 130 and the substrate 110. The reflective layer 150is configured to reflect the light downward from the light-emittinglayer 131 back above the light-emitting layer 131. However, the lightbeams with larger incident angles, such as light R₁ and light R₂, wouldbe gradually absorbed by the light-emitting layer 131 after passing backand forth through the light-emitting layer 131 due to the total internalreflection, and consequently the brightness and the luminous efficiencyof the light-emitting diode would be reduced. FIG. 1B is a schematicdiagram of another conventional light-emitting diode. As shown in FIG.1B, the light-emitting diode includes a transparent substrate 120 and amulti-layer epitaxial structure 130 having a light-emitting layer 131.When the light from the light-emitting layer 131 is reflected at thebottom of the transparent substrate 120 and travels to the sides of thetransparent substrate 120, some light beams (such as light R₃) would bereflected back inside the light-emitting diode because its incidentangle θ₁ is larger than the critical angle θ_(c), and have more chancesof being absorbed by the light-emitting layer 131. Therefore thebrightness and the luminous efficiency of the light-emitting diode arereduced.

Consequently, it is necessary to provide a light-emitting diode and amethod of forming the same capable of reducing the times the lightpassing through the light-emitting layer.

SUMMARY OF THE INVENTION

The present invention provides a light-emitting device having atransparent adhesive layer, including a transparent substrate capable ofimproving the brightness and a first and a second electrodes on the sameside.

In one embodiment, the present invention provides a light-emittingdevice including a transparent substrate, a transparent adhesive layeron the transparent substrate, a multi-layer epitaxial structure on thetransparent adhesive layer, the multi-layer epitaxial structureincluding a light-emitting layer, a first electrode on the transparentadhesive layer, and a second electrode on the multi-layer epitaxialstructure. The transparent substrate has a first surface facing thetransparent adhesive layer and a second surface opposite to the firstsurface, and the ratio of the area of the second surface to the area ofthe light-emitting layer is not less than 1.6.

In another embodiment, the present invention provides a light-emittingdevice including a transparent substrate, a transparent adhesive layeron the transparent substrate, a multi-layer epitaxial structure on thetransparent adhesive layer, a first electrode on the transparentadhesive layer, and a second electrode on the multi-layer epitaxialstructure, wherein the transparent substrate has a first surfacecontacting the transparent adhesive layer and a second surface oppositeto the first surface, and the area of the second surface is larger thanthat of the first surface.

The present invention further provides a method of forming alight-emitting device. The multi-layer epitaxial structure is attachedto the transparent substrate through the transparent adhesive layer andthen diced to obtain light-emitting devices with improved brightness.

In one embodiment, the method includes a step of providing a temporarysubstrate having a multi-layer epitaxial structure and a firsttransparent conductive layer formed on the temporary substrate, and astep of cutting the temporary substrate to form a first die, the firstdice including a portion of the multi-layer epitaxial structure, aportion of the first transparent conductive layer, and a portion of thetemporary substrate. The method also includes the step of providing atransparent substrate having a transparent adhesive layer formed on thetransparent substrate, and a step of attaching the first die on thetransparent adhesive layer. The transparent substrate is then cut toform a second die, the second die includes at least one of the firstdie, a portion of the transparent adhesive layer, and a portion of thetransparent substrate. The transparent substrate of the second die has afirst surface contacting the transparent adhesive layer and a secondsurface opposite to the first surface, and the ratio of the area of thesecond surface to that of a light-emitting layer of the multi-layerepitaxial structure is not less than 1.6.

In another embodiment, the method includes the step of providing atransparent substrate having a light-emitting element on the transparentsubstrate. The light-emitting element includes a transparent adhesivelayer on the transparent substrate, a multi-layer epitaxial structure onthe transparent adhesive layer, a first electrode on the transparentadhesive layer, and a second electrode on the multi-layer epitaxialstructure. The transparent substrate is then cut to make the ratio ofthe area of a second surface of the transparent substrate distant fromthe transparent adhesive layer to the area of a light-emitting layer ofthe multi-layer epitaxial structure is not less than 1.6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are schematic diagrams of conventional light emittingdiodes;

FIGS. 2A-2C are schematic diagrams of light emitting devices accordingto the present invention;

FIGS. 3-6 are schematic diagrams showing the steps of forming a lightemitting devices according to the present invention; and

FIGS. 7A-7C are schematic diagrams showing different cutting methodsimplemented in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention would be illustratedwith reference to the appended drawings. It should be noticed that, topresent this invention clearly, the layers and elements in the drawingsare not depicted to scale, and the known components, materials, andprocessing techniques would be omitted below to avoid obscuring theteachings of the present invention.

FIGS. 2A-2C show preferred embodiments of the present invention. Alight-emitting device 200 according to the present invention includes atransparent substrate 210, a transparent adhesive layer 220 on thetransparent substrate 210, and a first transparent conductive layer 230on the transparent adhesive layer 220. The material of the transparentsubstrate 210 includes but not limited to glass, sapphire, SiC, GaP,GaAsP, and ZnSe. The transparent adhesive layer 220 can have a materialincluding but not limited to spin-on glasses, silicone,Benzocyclobutene(BCB), Epoxy, polyimide, and Perfluorocyclobutane(PFCB).The first transparent conductive layer 230 can be made of a materialincluding but not limited to indium tin oxide, cadmium tin oxide, zincoxide, and zinc tin oxide.

Moreover, as shown in FIGS. 2A-2C, the light-emitting device 200according to the present invention further includes a multi-layerepitaxial structure 240 and a first electrode 250 on the firsttransparent conductive layer 230, and a second electrode 260 on themulti-layer epitaxial structure 240. A trench 270 may be optionallyformed between the first electrode 250 and the multi-layer epitaxialstructure 240. The multi-layer epitaxial structure 240 includes a firstcontact layer 241, a first confinement layer 242, a light-emitting layer243, a second confinement layer 244, and a second contact layer 245. Toform a good ohmic contact with the second electrode 260, a secondtransparent conductive layer 261 capable of spreading currents may beoptionally formed between the second electrode 260 and the secondcontact layer 245. The second transparent conductive layer 261 can bemade of a material including but not limited to indium tin oxide,cadmium tin oxide, zinc oxide, and zinc tin oxide. The first contactlayer 241 and the second contact layer 245 can be independently made ofmaterials including but not limited to GaP, GaAs, and GaAsP. The firstconfinement layer 242, the first light-emitting layer 243, and thesecond confinement layer 244 can be made of materials including AlGaInP.The first electrode 250 and the second electrode 260 can be respectivelymade of a material including but not limited to Au, Al, Pt, Cr, and Ti.In the structures shown in FIGS. 2A-2C, the transparent substrate 210has a first surface 211 contacting with the transparent adhesive layer220 and a second surface 212 opposite to the first surface 211. However,it should be noticed that, the area of the second surface 212 is largerthan that of the light-emitting layer 243.

In the exemplary embodiment of FIG. 2A, the area of the second surface212 is larger than that of the light-emitting layer 243. As shown inFIG. 2A, the second surface 212 of the transparent substrate has an areaessentially equal to that of the first surface 211, and the areas of thefirst surface 211 and the second surface 212 are larger than the area ofthe light-emitting layer 243. Therefore, the first surface 211 of thetransparent substrate would form an exposed portion, “A”, not coveredwith the light-emitting layer 243. The exposed portion, “A”, should atleast not be covered with the light-emitting layer 243. For example, theexposed portion, “A”, in the figure is not covered with the multi-layerepitaxial structure 240, the first transparent conductive layer 230, andthe transparent adhesive layer 220. The size of the exposed portion “A”can be decided upon the area ratio of the first surface 211 to thelight-emitting layer 243, the second surface 212 to the light-emittinglayer 243, or both of them, and a preferred area ratio is not less than1.6. The structure having the exposed portion, “A”, can increase thebrightness of the light-emitting device 200. As shown in FIG. 2A, thelight R₄ traveling from the second surface 212 upward to the transparentsubstrate 210 leaves the light-emitting device 200 through the exposedportion, “A”, without passing through the light-emitting layer 243,hence the brightness is increased.

In another exemplary embodiment of FIG. 2B, the area of the secondsurface 212 is larger than that of the light-emitting layer 243. Asshown in the FIG. 2B, the area of the second surface 212 is greater thanthat of the first surface 211. More specifically, the cross-section ofthe transparent substrate 210 is like a trapezoid. This structure canincrease the brightness of the light-emitting device 200, because theincident angle θ₂ of the light R₅ traveling from the second surface 212to a side 213 of the transparent substrate 210 is smaller than thecritical angle θ_(c). In detail, α shown in FIG. 2B is the angle thatthe side 213 tilts to the multi-layer epitaxial structure 240. Theangle, “α”, changes the incident angle of the light R₅ from θ₁ in FIG.1B to θ₂ (namely θ₂=θ₁−α), and makes it smaller the critical angleθ_(c). Consequently, the light R₅ leaves the transparent substrate 210through the side 213, rather than be reflected back into the multi-layerepitaxial structure 240. Those who are skilled in the art shouldunderstand that the critical angle θ_(c) mentioned above depends on thematerial of the transparent substrate 210 and the environmental medium.Therefore, if the environmental medium is set, θ_(c) can be determinedby choosing an suitable transparent substrate 210, and the tilt angle,“α” is adjusted by changing the ratio of the area of the second surface212 to the area of the first surface 211 of the transparent substrate210. Taking the transparent sapphire substrate 210 for example, theratio of the area of the second surface 212 to that of the first surface211 is not less than 1.6, and preferably ranges between 4 and 20. Thethickness of the transparent substrate 210 is preferably between 50 to200 microns, more preferably between 80 to 150 microns.

In a further exemplary embodiment of FIG. 2C, the area of the secondsurface 212 is larger than that of the light-emitting layer 243. In thisembodiment, the second surface 212 is larger than the first surface 211,and the first surface 211 has an exposed portion, “A”. The ratio of thesecond surface 212 to the first surface 211 and the ratio of the secondsurface 212 to the light-emitting layer 243 are similar to thosementioned above.

Additionally, the light-emitting device 200 may further include areflective layer 280 on the second surface 212 of the transparentsubstrate 210 in view of demand. The reflective layer 280 shown in FIGS.2A-2C is, but not limited to, attached directly to the second surface212. The reflective layer 280 can be made of a material including butnot limited to Sn, Al, Au, Pt, An, Ge, Ag and the like. The reflectivelayer 280 can also be a distributed Bragg reflector (DBR) consisting ofoxides, and the oxides can be Al₂O₃, SiO₂, or TiO₂.

FIGS. 3-7 show preferred embodiments of forming the light emittingdevices according to the present invention.

As shown in FIG. 3, a temporary substrate 310 is provided, and amulti-layer epitaxial structure 240 is formed on the temporary substrate310. The steps of forming the multi-layer epitaxial structure 240includes sequentially forming a second contact layer 245, a secondconfinement layer 244, a light-emitting layer 243, a first confinementlayer 242, and a first contact layer 241 on the temporary substrate 310.Then a first transparent conductive layer 230 covering the multi-layerepitaxial structure 240 is formed. As shown in FIG. 3, an etch stoplayer 320 is provided between the multi-layer epitaxial structure 240and temporary substrate 310 to prevent the multi-layer epitaxialstructure 240 from damages caused by over etching in subsequent removalof the temporary substrate 310. Preferably, the etch stop layer 320 hasan etching rate lower than that of the temporary substrate 310.

After forming the multi-layer epitaxial structure 240 and the firsttransparent conductive layer 230 on the temporary substrate 310, thetemporary substrate 310 is cut to form a plurality of first dices 400.As shown in FIG. 4, the first dice 400 includes a portion of themulti-layer epitaxial structure 240, a portion of the first transparentconductive layer 230, and a portion of the temporary substrate 310. Thecutting step can be performed by use of a diamond tool or a laser tool.

Then, as shown in FIG. 5, the first dice 400 is attached to thetransparent substrate 210. A transparent adhesive layer 220 is formed inadvance on the first surface 211 of the transparent substrate 210 forbonding the first dice 400 to the transparent substrate 210. Moreover, areflective layer 280 may be optionally disposed on the second surface212 of the transparent substrate 210. The material of the reflectivelayer 280 is similar to those mentioned above.

Subsequently, as shown in FIG. 6, the surplus transparent adhesive layer220 exposed on the transparent substrate 210 is removed, and thetemporary substrate 310 is then removed. If the temporary substrate 310is made of GaAs, it can be removed by a chemical etchant solution suchas 5H₃PO₃:3H₂O₂:3H₂O or NH₄OH:35H₂O₂. After removing the temporarysubstrate 310, the etch stop layer 320 is further removed.

Then, structures as shown in FIGS. 7A-7C can be formed by conventionalprocesses, such as deposition, lithography and etching. In detail, themulti-layer epitaxial structure 240 is selectively etched to expose theunderlying first transparent conductive layer 230. Subsequently, atrench 270 as shown in FIGS. 7A-7C is formed, a first electrode 250 isformed on the first transparent conductive layer 230, and a secondelectrode 260 is formed on the multi-layer epitaxial structure 240. Thetrench 270 isolates the multi-layer epitaxial structure 240 from thefirst electrode 250. The first electrode 250 and the second electrode260 are formed on the same side of the transparent substrate 210.Additionally, a second transparent conductive layer 261 capable ofspreading currents may be optionally formed between the second electrode260 and the second contact layer 245. The second transparent conductivelayer 261 forms a good ohmic contact with the second electrode 260. Thesecond transparent conductive layer 261 can be made of a material of thefirst transparent conductive layer 230 as mentioned above.

Next, the transparent substrate 210 is cut to form a plurality of seconddice 200 (namely the light-emitting devices 200). The dotted lines inFIGS. 7A-7C respectively illustrate different cutting manners forobtaining the light-emitting devices 200 shown in FIGS. 2A-2C. Aftercutting, the second dice 200 includes the first dice 400, a portion ofthe transparent adhesive layer 220, and a portion of the transparentsubstrate 210. During cutting, it should be noticed that, thetransparent substrate 210 of thus formed second dice 200 has a firstsurface 211 contacting with the transparent adhesive layer 220 and asecond surface 212 opposite to the first surface 211, and the area ofthe second surface 212 is larger than that of the light-emitting layer243. The cutting manner of FIG. 7A exposes a portion, “A”, of thetransparent substrate 210 of the second dice 200. The cutting manner ofFIG. 7B makes the second surface 212 of the transparent substrate 210 ofthe second dice 200 be larger than the first surface 211 without theportion, “A”. The cutting manner of FIG. 7C creates the feature that thesecond surface 212 of the transparent substrate 210 of the second dice200 is larger than the first surface 211 with the portion, “A”. Thecutting can be performed by wafer dicing equipments with a diamond toolor a laser tool. To reduce the heat produced by cutting and take awaythe debris, water with a constant amount at a given pressure may belaterally introduced along the rotating direction of the diamond toolduring the cutting step.

The detailed description of the above preferred embodiments is todescribe the features and spirit of the present invention more clearly,and is not intended to limit the scope of the present invention. Thescope of the present invention should be most broadly explainedaccording to the foregoing description and includes all possiblevariations and equivalents.

We claim:
 1. A method of forming a light-emitting device, comprising:providing a temporary substrate having a multi-layer epitaxial structureand a first transparent conductive layer thereon; cutting said temporarysubstrate to form a first dice comprising a portion of each of saidmulti-layer epitaxial structure, said first transparent conductivelayer, and said temporary substrate; providing a transparent substratehaving a transparent adhesive layer on said transparent substrate;attaching said first dice to said transparent adhesive layer; andcutting said transparent substrate to form a second dice comprising saidfirst dice, a portion of each of said transparent adhesive layer andsaid transparent substrate, wherein said transparent substrate of saidsecond dice has a first surface facing said transparent adhesive layerand a second surface opposite to said first surface, the ratio of thearea of said second surface to the area of a light-emitting layer ofsaid second dice is not less than 1.6.
 2. The method according to claim1, further comprising removing said temporary substrate of said firstdice after attaching said first dice on said transparent adhesive layer.3. The method according to claim 2, after removing said temporarysubstrate of said first dice, further comprising: etching a portion ofsaid multi-layer epitaxial structure of said first dice to expose saidfirst transparent conductive layer; forming a first electrode on saidexposed first transparent conductive layer; and forming a secondelectrode on said multi-layer epitaxial structure of said first dice. 4.The method according to claim 1, after said step of attaching said firstdice to said transparent adhesive layer, further comprising removingsaid transparent adhesive layer not covered with said first dice therebyexposing a portion of the transparent substrate.
 5. The method accordingto claim 1, further comprising providing a reflective layer on saidsecond surface.
 6. The method according to claim 1, wherein said cuttingsteps are performed by use of a diamond tool or a laser tool.
 7. Themethod according to claim 1, wherein the area of said second surface islarger than the area of said first surface.
 8. The method according toclaim 1, wherein the ratio of the area of said second surface to thearea of said first surface ranges between 4 and
 20. 9. The methodaccording to claim 3, further comprising a step of forming a secondtransparent conductive layer between said second electrode and saidmulti-layer epitaxial structure of said first dice.